I was wondering whether anyone had ever wrapped one of those emergency foil blankets around their sleeping pad to reflect body heat back up towards the bodies core during the night. If so, what difference did it make?

It will make no improvement in IR heat retention; it will cause the sleeping bag to loose conduction insulation because it won't pass moisture to the air; and will only prevent convection heat loss if you don't already have a conventional shelter such as a tent, bivy, or tarp.

A foil blanket is designed to reflect infra red heat. Any conventional sleeping bag insulation loft higher than 3/4 to 1" (60F rating) will already absorb the vast majority of your IR heat and so there will be little or nothing for the foil to reflect back to your body.

A foil blanket is useful for situations in which you don't have 3/4 to 1" of fiber insulation; you want to maximize the heat from a fire by reflecting it to your back side; you need to block the suns heat and you have an air gap greater than 1" you and the blanket; or you need to block the wind (convection heat loss) and don't have a conventional shelter.

Foil placed between the bottom of the sleeping bag and the top of the pad will not effectively improve the insulation value of the pad or the bag.

This is because the bottom of the sleeping bag will be compressed by your body weight and so there isn't an effective air gap for the foil to reflect infrared radiation. Most of your body heat will be passed through the foil via conductance to the pad.

So, it seems that there needs to be a gap between the body and the foil to prevent heat simply being conducted through the foil instead of being reflected back.

I had been thinking about ways to improve bottom insulation when using a top bag without adding massively to weight carried. I usually have a foil blanket in my pack but have neve used it for this purpose and don't have a another trip away planned for a while.

I did like the look of the balloon bed, maybe that would provide the air gap required if the foil was placed on the floor beneath the bed? Or maybe just stick with a pad and forget about the foil.

I used to sleep with a cut down Outdoor Blanket as a ground cloth. The is a 3-layer laminate with a reflective metalized side. I used it without a sleeping pad with the reflective side up. I always preferred these to the foil blankets where the metal coating peels off after a few uses.

Never noticed any special warming effects from merely laying on it. However, during the 3:00 a.m. chill, it was warming to pull it around you. Don't know if it was trapping radiant heat or merely acting as a VBL. I suspect the latter.

If you are hearing impared it should work OK, mostly by being windproof. If your hearing is up to par, the rattle, crinkle, snap, crackle and pop of the foil blanket will remind you of what sleep deprivation is all about.